The science of hot air rising

Since energy myths are front and center at the moment I would like to discuss a single myth, the science of hot air rising. Actually, I have for my own purposes upgraded this one to an energy legend, as it has proved extremely difficult to correct. I'm assuming it is wrong but I'm certainly open to all opinions.

There are several areas in our energy business where this is important, attic venting, stack effect, convection, and chimney draft to name the obvious. The legend as I see it is that people have observed warm air moving up for so long that there is a belief that warm air has some inert power of its own. Statements like "the warm air will rise and exit the upper vents and pull the cold air in the lower vents", implies that the warm air initiated that process and as a result not only pulled the cold air in through the soffits, but additionally air from the house as well. My belief is, the opposite is true. The cold air pushes its way into the attic and forces the warm air up and out the upper vents based upon the principles of buoyancy. Here is a simple article by April Holladay that explains this invading cold air process very well is:http://www.usatoday.com/tech/columnist/aprilholladay/2005-02-18-won...

At first glance this appears to be just a simple statement of what everyone sees in the real world. But the concept that cold air is the driving force becomes important in properly explaining the other, above, modes of air movement. As energy professionals I believe it is important that we determine the truth about this legend and learn to state it correctly so future generations will not be led to believe that there is magic in warm air.

David, be clear that there is no intent to deceive anyone here, quite the opposite. But the variety of issues the various posters propose to complicate such a simple process does make the thread wander.

But, let's try to avoid a fight and try to take the points you raise one at a time.

You did mention gravity, but then you said "we're looking neither at pushing nor pulling, but some combination of changing places and diffusion." Can you explain "changing places" and how that occurs without pushing and pulling?

As for "Air has no ability to maintain differences within a volume." the houses we work with are not good examples as we add heat to keep the process going. Without heating or cooling, I agree that temperature differences would equalize within a home and soon between the home and outside due to leakage.

There is still only one topic, "the science of hot air rising" but getting that message across has been challenging. I would have thought that Dr Mills post would have been sufficient. Or Professor Summer's video. Or even my diagrams that spell out how stack effect pressures come about without hot air having any special powers.

My post on Thursday was an attempt to move forward with the conclusions that have gained acceptance, certainly not universally, but enough that I felt it was time to approach the benefits of this new understanding. But I have no objections to continuing the debate as long as we keep it friendly and I'm sure HEP will grant us the space to do so.

Ok, Hot air CANNOT rise by itself, it is just lighter air. We need to conclude this and move on to what this means for our understanding of draft, convection, stack effect, ventilation, and more. I suspect many will still want to contest this conclusion, here is the place. For those who wish to follow and contribute to explaining why we see hot air moving up in so many different ways, we will continue on a new thread: http://homeenergypros.lbl.gov/forum/topics/who-knew-hot-air-could-b...

For those still in doubt, simply remove heat from the discussion for a moment and consider a lighter gas in the presence of a heavier gas, Helium for example. The lighter gas will be pushed up by the heavier gas. Try another. Release a cork at the bottom of a swimming pool and it will race to the top. Examine it carefully and it is just a cork, so why did it go up. The heavier fluid around it "pushed" it up, in exactly the same way that cold air "pushes" our warm air up.

One more thought for those who concluded that changing our view and our description on why hot air moves up would just make explaining the process unnecessarily complex. It's ironical that we often talk about the need to change old bad habits to keep pace with the new energy efficiency methods, yet when something comes along that needs to be changed and someone doesn't like it, suddenly we aren't supposed to change. I am not trying to change science here, I'm just looking for a better way to describe air movement in a way that does not lead some to conclude that "hot air" has magical powers. The questions that will be addressed on the thread shown above cannot be correctly answered by "hot air rises".

Guys, I think one of the problems we recognize and are trying to avoid is myopic oversimplification.

But isn't the idea that a force down is the only force acting on our atmosphere just that? What about the counterbalancing centrifugal force without which we would all be pancakes? We've all played with those child toys with ball, string, and stick, that when spinning fast enough keep the balls at the end of the strings irrespective of gravity.

I really see this not as pushing up or down, more a combination or blend of forces and relative densities that allow for buoyancy.

In some cases gravity and density win, in some centrifugal force and lack of density win.

I'm smiling Ted. I think you are avoiding "myopic oversimplification" too hard. One of the primary objectives here is to find a simple explanation and gravity is certainly the dominant force involved. None of the professionals calculating our stack effect seem to include the forces you speak of.

In reality, we are not concerned about the exact gravitational force, but the difference between the weight of a column of warm (or cold) air inside our homes vs one outside.

lol. Probably gets my focus because it's something we all seem to run into so frequently! This is one major challenge of being in a really new profession that is just beginning to get to the main stream.

Isn't that the whole point of this thread, help us communicate a really important energy issue that laypeople tend to know nothing about, and give way to little import once we do explain it? I mean, we aren't doing this for ourselves, it's to come up with ever better translations for NON building scientists - or have I again gone off into left field...

As I learned this stuff I did it with friends in the HVAC field, and friends in the insulation field. It was amazing to all of us how our long held perspectives were "corrected".

Think of all the HVAC and Insulation guys who have yet to learn these things! (I see recently insulated homes all the time with big blower door numbers, big energy bills, and air leaking all around and past very attractively installed insulation. I see houses with brand new furnaces and air conditioners that are grossly oversized to both house and to duct work - static's well above .5 - and homeowners reporting no savings from their new "high" efficiency furnace.)

I agree that better wording for our consumers is the bottom line, but with this hot air issue we first need to bring the energy professionals into agreement. It wasn't until a few months ago that I questioned ye-old short-circuit theory, the question about removing or leaving those gable vents. Once I looked at it, the common description that the hot air exits the ridge vent and pulls in its replacement air from below and thus the soffit area would be short-circuited by the gable path didn't seem right. So I went looking for the forces behind static attic ventilation. The answer turned out to be, the cold air pushes in through the soffits and forces the warm air up and out the ridge vent, a process that would essentially be unaffected by those gable vents. There are reasons to consider removing gable vents, but short-circuiting between ridge and soffit is not one of them. When I went to explain that on an Energy Auditor only forum, I was nailed to a cross by mt peers for my opinion that hot air does not rise by itself.

Much reading and much discussion later, I can now return to the short-circuit myth and other areas of confusion and apply a new line of thinking, one that has helped me greatly and I'm guessing will help others. In a world of tighter and tighter homes, understanding ventilation and stack effect will be essential.

>> Cold air invades the lower portions of our homes and pushes the warm air up and out.

If warm air does not have any intrinsic motive power to rise, then cold air cannot have any intrinsic motive power to push the warm air up.

Clearly the air is moving due to pressure differentials, which can be analyzed into gravity/buoyancy of individual "packets" of air within a single volume of air, or within stacks when the volume of air is divided into vertical columns of air by partitions.

In the common person's experience the extreme situations are noticed and remembered: hot air is noticed to rise above fire as with a candle or fireplace. Cold air is noticed to fall as with the cold air falling out of the refrigerator.

If we want to explain these things to the common person, then we must begin with what s/he understands intuitively: hot air rises, cold air falls, and then explain how the situation under consideration is the same as, or different than that.

The effective words to do this with are "hot air USUALLY rises, cold air USUALLY falls." This helps open the mind of the person to wonder when hot air does not rise, which then (if their mind does open) is our opportunity to deepen their understanding. I can often get this to happen by saying, "hot air USUALLY rises" and then I simply stop talking and let it sink in until they say something, which is sometimes the question, "when does hot air not rise?"

If a person did not learn these things about the real world around them by the time they were five or eight years old, (playing with rubber duckies in the bathtub, swimming in the crick, or blowing bubbles on a hot summer afternoon in and out of the shade of an elm tree) then they will only be confused by eleventh grade physics, or any explanation from an energy auditor. The huge problem we have to deal with now is that there are a couple of generations in which many people did not play with rubber duckies or swim in the crick because they were too "germy" or they were watching TV, or playing game boy and listening to Sony-babble on their walkman or ipod instead of blowing bubbles and learning about how the real world works. They are lost in an alternate reality of consumerism, and the closest they will come to understanding how their house works is to sign the $9,000 contract that is handed to them by the replacement window pirate (who disguised himself as an energy auditor).

I take hope though, perhaps a few will gain some understanding from a real energy auditor or a knowledgeable tradesperson.

There is the obvious gap between what energy auditors should know and how they should explain things to their customers and every customer is going to be different. But if you were to ask 100 energy auditors why hot air goes up, you would get a surprisingly large number who believe it is just because hot air rises. In reality, hot air is being pulled down just like cold air and everything else. That negative pressure we see in our basements isn't the warm air trying to move upwards, but the lesser weight of the stack of air inside the house. If you look at a concrete wall on a walkout basement, the warm air on the inside is pushing on the wall trying to get out at the same time the cold outside air is pushing to get in. Provide a path and the cold air wins because it is pushing harder. That's the barometric pressure difference between inside and out.

Add to the descriptions above the math that is needed to quantify these pressures and it becomes necessary to know the science. As more and more people become aware of this distinction, it is my hope that a few select phrases will evolve that do a proper job of conveying the message we want, yet avoid any inference that warm air is self powered. I wonder how the Brits explain it? They've been at this a bit longer than we yanks.

John, I've thought about something you hit upon in your reply regarding why "how the world works" seems to be lost on many people. Back when life for most human beings was more or less hand to mouth, knowing how the world worked made the difference between life or death, or between a decent life and a hard one. As we advanced technically, there seems a concurrent force that moved many away from this razor sharp grasp of reality.

In our day it's possible to live cradle to grave and have no understanding of essential physics, or agriculture, or an advanced understanding of technology. How so? How many among us have careers that fit a specific niche, which seldom forces that same person to stretch their boundaries? Even among specialized fields, I am at times amazed how someone who is genuinely an electrical engineer can be completely flustered by a residential a/c control circuit. Maybe I'm missing something, but it strikes me that if one is an electrical engineer, the knowledge required to attain that status, and the principles involved therein, can be readily spotted across various platforms of related fields.

I suppose where I'm going is that perhaps many of us are victims of our own success...we are technologically advanced to where the necessity to possess a variety of knowledge in related areas isn't as strong as it once was. An example would be my own field of HVAC and refrigeration. When the field was young, most accomplished technicians (more likely called "mechanics" back then) could work on most anything that involved refrigeration, from a simple window unit to a chiller. The reality of their time was they had no choice. Either be able to work on all of it or lose out to the guy who could.

Now, everything is specialized. A residential service technician can spend his entire career excelling in residential service, and never touch a chiller or supermarket rack system. Same holds true for the chiller guy. Rare is the tech that is completely comfortable with both realms. Such a tech at heart would realize it's essentially the same principles in play; only the complexity of execution involved varies.

Tying this all into our topic at hand, it will always be a challenge to explain the physics of building science to those who, primarily, have little interest in it beyond what they are told is needed to solve their problem. The curiosity (which is an underlying aspect of what I discussed above) is not there, mostly. If the curiosity is not there, the basis for expanding one's knowledge base beyond a comfort level is also not there. Translation: average building owner could probably care less about the actual physics behind air movement through a structure. To him, warm air rises and cold air falls; going beyond that is just getting into "geeky" territory.

Who can stand to benefit from this discussion is those of us who are in or involved with building science matters. We already have the curiosity. We already have various levels of training and education. What a discussion like this can do is help refine said knowledge/education and perhaps clear up misconceptions. I personally have benefited from this discussion as it has helped clarify certain conclusions I reached on my own, and dispelled others.

How one might go forward to translate this advanced understanding to an end user will fall within the realm of the "soft skill" level of the energy auditor, rater, or building scientist. It's a true talent to take rather complex subjects and make it not only basically understandable to a "layman", but also stoke the curiosity of this same person beyond mere courtesy. An example that comes to mind of who does this well is Dr. Joe Lstiburek. He can take a topic that on its surface is rather "dry", and make it seem, at minimum, damn interesting.